The metre used with the appropriate prefix is capable of measuring vast distances across the universe and the tiniest particles in the universe. These prefixes remove the need to use enormous numbers with lots of trailing zeroes. So why do astronomers continue to use astronomical units (AU), light years and parsecs? These non-standard units can be replaced by the metric unit of length, the metre, with the appropriate prefix.
The Astronomical Unit (AU) is the distance from the Earth to the Sun, roughly 150 million kilometres. An AU can be expressed as 150 gigametres.
A light year is the distance that light travels in a vacuum in one Julian year (365.25 days). The distance of a light year is roughly 9.46 trillion kilometres. A light year can be expressed as 9.46 petametres.
A parsec is roughly equal to 3.26 light-years, 206 265 astronomical units or 30.856 trillion kilometres. A parsec can be expressed as 30.856 petametres.
To demonstrate the full range of the metric system for greater distances than a kilometre, here are all the prefixes larger than “kilo” that can be used with the metre:
| Prefix + Unit Name | Symbol | Value |
|---|---|---|
| quettametre | Qm | 1030 metres |
| ronnametre | Rm | 1027 metres |
| yottametre | Ym | 1024 metres |
| zettametre | Zm | 1021 metres |
| exametre | Em | 1018 metres |
| petametre | Pm | 1015 metres |
| terametre | Tm | 1012 metres |
| gigametre | Gm | 109 metres |
| megametre | Mm | 106 metres |
Here are some interesting facts about the universe expressed in the metric system:
- The Andromeda Galaxy is about 23.65 zettametres from the Earth.
- The nearest large galaxy cluster, the Virgo Cluster, is about 510.84 zettametres from the Earth.
- The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way’s, is about 6.15 yottametres from the Earth.
- The galaxy filament Hercules–Corona Borealis Great Wall, currently the largest known structure in the universe, is about 92.71 yottametres across.
- The particle horizon (the boundary of the observable universe) has a radius of about 435.16 yottametres.
If yottametres are not long enough, astronomers can use two larger prefixes that were added to the metric system in 2022 and were formally approved by the BIPM. These prefixes are “ronna” and “quetta”. The metre and its prefixes can replace the AU, light year and parsec. It provides more than enough flexibility with a wide range of prefixes to cover the full range of distances used in astronomy.
Sources and further reading:
Metric Views 
For what it’s worth, I don’t think the light-year is used much anymore inside astronomy, only when communicating results to non-astronomers. Perhaps expressing distances in metres would help in this context, but then within the field there is a particular problem in that the current definition of metre experiences some wonkiness with distances relevant to astronomy, mostly because it assumes that the gravitational field is uniform which it very much is not in some cases. Using au and pc, in this case, at least gives some implicit out-of-band signal that you should be aware of some weird general relativity things going on.
The largest distances are usually expressed in z, redshift, a dimensionless number which has no easy conversion to SI units, prefices or no, again because of the uncertainty in the way the universe expands requires more than just redshift to compute, and there’s more than one way to compute a distance as well which all give different values. The values you quote for distances to very far away objects is just one way of converting z to a distance. z is what is actually being measured, too, so it’s entirely relevant.
The current SI prefices remain insufficient for mass. M_sol is a standard unit for stellar and larger astronomy, and it being 2e30 kg, the current prefices just barely fit, and the most massive things in the universe are millions of M_sol. Of course, smaller things are measured in M_Jup (2e27 kg), M_E (Earth, 6e24 kg), more rarely M_L (Moon, 7e22 kg) and even smaller things are usually expressed in kg directly.
Much like aviation and particle physics, the usage of non-SI units is partially to hide away some details that turn out to be useful somehow, or to introduce out-of-band signalling that would otherwise require more words to say – the fact that the number in the quantity is closer to 1 as a result is largely secondary. Depending on the field, formulae and constants may already have established numbers that prove difficult to dislodge, even if the unit that number is in is genuinely stupid, perhaps because that number is derived from a graph that was used to define the quantity initially. The Hubble constant is usually given in units of (km/s)/Mpc, which is of course Hz with a prefactor – though, if I did express a possible value of the Hubble constant as 2.27 aHz, is it any more enlightening?
LikeLike
I’m not so sure about using the metre for all astronomical measurements, because of the nature of the things to be measured.
First, the largest distances require factoring in the inherent expansion of the universe. Any expression of distance in the region of about the Gpc (~ e25 m) scale onward will need to take this into account seriously. Within the field this is sidestepped by expressing z (the redshift) directly, which is directly measured and reported as if it is a distance – you see something like “z = 0.5”, which is both an expression of distance (~59 Ym) and time (~160 Ps ago). However, it cannot be converted to a distance without making some assumptions about how the universe expanded and continues to expand, which the values you quoted and I used above definitely did. And unfortunately, while there is a canonical way to do so, journalists occasionally use technically meaningful but largely for-reference-only methods of doing the conversion. Though you may be happy to hear that at least within the field, the usage of ly is basically gone, though only because pc is now preferred.
Second, even in smaller distances using au ~ pc scales, it can be argued that it hides away the somewhat uncomfortable fact that in some cases general relativity can make the idea of measuring distances somewhat difficult. The current definition of the metre only takes special relativity into account, and assumes that the gravitational field is more or less constant, which may not be the case in certain extreme objects like black holes. Although the au, and so the pc, are now both defined as a constant number of metres, I argue that using “au” can be an out-of-band signal to be aware of GR wonkiness, much like how aviation retains the nautical mile and the foot as out-of-band signals that they are measures of distance along the ground and distance above the ground respectively, and are likely to continue the practice even if they change to km and m finally.
Finally, there is mass, which is not well served by the SI system at all. Astronomical units of mass include the solar mass (M_sol ~ 2e30 kg), the Jupiter mass (M_Jup ~ 2e27 kg), the Earth mass (M_E ~ 6e24 kg), and the lesser used lunar mass (M_L ~ 7e22 kg). Smaller objects still tend to be expressed using kg and scientific notation only. But with black holes being many thousands to millions of solar masses, and galaxies being heavier still, even the extended crop of prefices can’t cope.
There are other quantities like time and energy that are equally bothersome and expressed in correspondingly obscure units, like my perennial favourite, the foe (= 1e51 erg = 1e44 J), which is the energy scale emitted by a typical supernova. But on the other hand, some of the “bad units” are somewhat more meaningful than they initially look like, referring to a graph that discovered them in the first place. The admittedly silly unit (km/s)/Mpc is used to express the Hubble constant, which similar to the kWh, is just a simpler unit with a prefactor. However, I argue it’s less enlightening to see “2.2 aHz” in place of “70 km/s/Mpc”, just because a frequency can refer to a lot more things. Granted, units of (km/s)/Zm (say) would retain that semantic, though once again complications with regard to large-scale distances will have to be addressed once again in a way that Mpc can technically hide – by ignoring the strict definition and re-scaling the longer distances based on z and the like.
LikeLike
Unless we are saying that the litre is not included in the metric system, by definition the astronomical unit must be part of the metric system. The astronomical unit (symbol au) is listed in the SI brochure, alongside the litre, hectare and tonne, as being “accepted for use with the SI”. It is defined as being equal to 149 597 870 700 metres.
Therefore, would it be better for the article to say that, “SI can meet all astronomers’ needs”? (rather than “Metric system can meet all astronomers’ needs”). I agree that SI units are preferable to non-SI units.
Also, to be consistent with the earlier Metric Views article advising against using huge numbers with prefixed units like kilometres, would it be better to say:
“150 billion metres”, rather than “150 million kilometres”,”9.46 quadrillion metres”, rather than “9.46 trillion kilometres” and”30.856 quadrillion metres”, rather than “30.856 trillion kilometres”.
https://metricviews.uk/2023/08/17/nasa-voyager-2-reports-make-poor-use-of-the-metric-system/
LikeLiked by 1 person
In my personal view astronomy is the most back-wards of all of the physical sciences. The fundamental theory of the big bang can not actually be proven to be true, unless you go back to the beginning to observe it. They say the universe is expanding, but how does and infinite universe expand? The observable universe expands but only in the sense that due to the constraints of the speed of light, our ability to see beyond the barrier set up by the big bang theory is limited by the age of the universe just under 14 Ga old. So said observable universe would expand at a rate of 299.792 458 Mm/s.
Unfortunately, the big bang theory has such a tight hold on academia that alternate theories are never given a chance to be heard. So it is not a surprise that these same “forces” are keeping SI out of astronomy.
LikeLike
m said:
“Also, to be consistent with the earlier Metric Views article advising against using huge numbers with prefixed units like kilometres, would it be better to say:
“150 billion metres”, rather than “150 million kilometres”,”9.46 quadrillion metres”, rather than “9.46 trillion kilometres” and”30.856 quadrillion metres”, rather than “30.856 trillion kilometres”.”
I completely disagree with this. I loathe the use of counting words with SI units. In fact the restriction of units to just the original prefixes surrounding unity degrades SI to being just a clone of FFU.
Counting words are not coherent as there are two forms, the short and the long. Confusion could result if the wrong counting word is used based on which format is intended. SI prefixes not only eliminate the confusion, they make the number less cluttered. 150 gigametres (150 Gm) sounds much more organised than 150 million metres.
In astronomy, distances from the planets to the moons would be measured in megametres, from the sun to the inner planets on gigametres, to the outer planets in terametres, the distance to the nearest stars, like alpha centauri, in petametres and large distance within a galaxy in exametres. The diameter of the milky way galaxy is 1 Zm and the observable universe is 880 Ym in diameter. As far as distance is concerned, the entire distance spectrum can be expressed with the present range of prefixes.
This topic has already been touched upon by Metric Maven, and two of his essays can be read here:
https://themetricmaven.com/?s=astronomy&submit=Search
LikeLiked by 1 person
@DanielThe article gives an equivalent for a value in gigametres in terms of “million kilometres”. As I pointed out, the author himself criticises this practice in an earlier article. My suggestion was that if an equivalent for 150 gigametres is given, then it should not be in terms of million kilometres. I do not understand why you say that you “completely disagree with this”, given what you go on to say about the use of “original prefixes”.
My proposed edit would be consistent with the fact that the value of the “giga” prefix is normally described as being equal to “billion”, not as “million kilo”.
Personally, I would not have included any equivalent for the value in gigametres, as most readers would be familiar with the value of the “giga” prefix being equal to “billion”. However, maybe I’m wrong about this given that you say, “150 gigametres (150 Gm) sounds much more organised than 150 million metres”.
LikeLike
While the “fundamental theory” of the Big Bang cannot be proven to be true, the same can be said for any description of what happened in the past, and anyway as of now the current theories have nothing to say about times earlier than 1e-43 s after what theory predicts is the beginning of the universe – only that up until then, things were closer to each other than they are now, and that is a direct observable, by seeing structures in the CMWB that are now causally disconnected but are similar enough that they must have been causally connected in the past.
Second, the metric expansion of the universe is intrinsic; it does not need to expand into anything, and there’s therefore no contradiction to an infinite universe expanding, even though an infinite universe isn’t required by theory in any case. Furthermore, it is not subject to the speed of light at all and can expand or contract arbitrarily quickly. One can make a (very) rough analogy to monetary inflation: a diamond is $1000 one year, $1500 the next, even though nothing about the diamond has changed – it’s as if a metre there is somehow “worth less” than a metre here.
(The expansion of the observable universe is also, by virtue of the metric expansion of space, not restricted to growing at c; it’s c plus however much the space it “rides on” expands relative to us.)
Bringing it all together, it is because of this metric expansion of space that expressing things in metric brings in complications that would otherwise be hidden away by such legacy units. Two very far away things can be said to be three or four distances away from each other, whose quantities are one or two orders of magnitude apart from each other. All of them say something meaningful, none of them tell the entire story, though one does come close. Astronomy and cosmology in particular poses challenges beyond what ordinary metrology can (should?) deal with, and just like in other places, the usage of units that seemingly have the same dimension as SI units but with prefactors can (should) signal readers of that fact.
LikeLike
One can define any unit of measurement in metric terms. That does not make it a metric unit in my books. A unit that I would call metric needs to be derived from the metre in some way. Thus a litre is 1 000 cubic centimetres. A kilogram is a litre of water. OK, it has a latter-day official definition, but that is its basis. An astronimical unit is arbitrary, being based on a natural physical value. Hence it is not strictly a metric unit. Not all SI units are metric.
If astronomers choose to use units like astronomical unit, parsec, light year, then that is their prerogative. It is not for me to object.
I avoid using units like billion, trillion, etc., wherever possible. There is ambiguity over their definition. A billion is a million squared in my books. I don’t know why the media and officialdom have chosen to use the less-logical American definitions. My guess is that they don’t know themselves.
I am not interested in learning prefixes above tera. It is unlikely that I will ever need to handle quantities greater than this. If I should need to do so, the exponent notation (power of 10) is unambiguous and mathematically easy to process.
LikeLike
Since 1960, the terms ‘metric system’ and ‘SI’ are synonyms. The international office of weights and measures (BIPM) states:
“…International System of Units (SI), which is the modern form of the metric system“
LikeLike